This invention relates to the field of AM stereo broadcasting and, more particularly, to a system wherein the high frequency modulating signals are modified before transmission and restored after reception.
The ideal AM stereo signal is pure quadrature but since the envelope of any AM broadcast signal is required to be compatible with present monophonic receivers (1+L+R), pure quadrature cannot be used on the broadcast band. One system for transmitting two information signals on a single amplitude modulated carrier with no distortion in monophonic receivers was disclosed in a U.S. Pat. No. 4,218,586, assigned to the assignee of the present invention. In this system the envelope is always 1+L+R, the monophonic signal, and the instantaneous phase angle .phi. is the arc tangent of [(L-R)/(1+L+R)]. This system is termed C-QUAM.TM. (compatible quadrature amplitude modulation). In stereophonic receivers a correction signal having the value cosine .phi. is used to recover the original audio signals. This system is completely satisfactory for all normal program material but it is possible for extreme program conditions to cause a slight increase in adjacent channel interference. An example would be the transmission of a high frequency signal in one channel only with a high modulation level, an unlikely situation in actual programming. Naturally, the problem is more likely to arise in countries that use narrow inter-channel spacings or where the allowable radiation is severely restricted with regard to the presence of sideband components above the highest audio frequency.
Attempts have been made to solve the problem by controlling or restricting the L-R difference signal levels at high audio frequencies, but this has the effect of reducing the high frequency stereo separation at the receiver. In another system which was based on the C-QUAM.TM. system referenced above, the transmitted signal was a compatible quadrature signal up to a predetermined frequency and a pure quadrature signal above that frequency. This system was usable but not completely satisfactory since it resulted in distortion around the crossover frequency, and even some distortion in the high frequency quadrature signal due to the presence of the low frequency signals. In some countries, however, the best practical signal is still a compatible signal for all normal program material, but changing to pure quadrature where necessary to eliminate adjacent channel interference.